Biomaterials have been used as implants in the field of spine, orthopaedics and dentistry including trauma, fracture repair, reconstructive surgery and alveolar ridge reconstruction, for over a century. Although metal implants have been the predominant implants of choice for these types of load-bearing applications, additional ceramics and nonresorbable polymeric materials have been employed within the last twenty-five years due to their biocompatibility and physical properties.
Polyetheretherketone (PEEK) is a biomaterial often used in medical implants. For example, PEEK can be molded into preselected shapes that possess desirable load-bearing properties. PEEK is a thermoplastic with excellent mechanical properties, including a Young's modulus of about 3.6 GPa and a tensile strength of about 100 MPa. PEEK is semi-crystalline, melts at about 340° C., and is resistant to thermal degradation. Such thermoplastic materials, however, are not bioactive, osteoproductive, or osteoconductive.
Conventional processes do not effectively provide a material or a method of making the material which combines a biocompatible polymer such as PEEK with a bioactive glass having a particle size larger than one micron. Furthermore, these processes do not incorporate a material or disclose a method of making a bioactive implant material which combines PEEK and bioactive glass of various particle sizes and which has the appropriate structural and mechanical properties to withstand the stresses necessary for use in spinal and orthopaedic implants.
A combination of polymers including PEEK and Combeite glass-ceramic, a bioactive glass, has generally been described in U.S. Pat. No. 5,681,872; U.S. Pat. No. 5,914,356; and U.S. Pat. No. 6,987,136, each of which is assigned to the assignee of the present invention and is incorporated in this document by reference in its entirety. It has been discovered, however, that conventional methods of combining polyaryletherketones, such as PEEK, and bioactive glasses, such as Combeite bioactive glass-ceramic, for example, combination using a screw extruder, results in a reaction between the PEEK and the Combeite glass-ceramic that forms a material having properties which inhibit extruder functioning. In some instances, the reaction makes combining bioactive materials, such as glass, ceramics, and glass-ceramics, with PEEK, or similar polymers of the polyaryletherketone family, a challenge using conventional processing. Attempts to combine PEEK and a bioactive glass without the use of a screw extruder have been made. For example, International Patent Publication WO 2008/039488, which is assigned to the assignee of the present invention, discloses a method of mixing PEEK and a bioactive glass followed by a compression molding step to form an article. Although this process successfully produces a bioactive article, the homogeneity of the bioactive article, in part, relies upon the PEEK and the bioactive glass being processed in powder form so that the starting particle size of the PEEK and the particle size of the bioactive glass are closely matched. Furthermore, compression molding methods such as this disclosed are not ideal for large scale bulk material preparation.
It is desirable, therefore, to have a process that successfully employs an extruder when producing bioactive composites such as, for example, PEEK and Combeite, because the equipment is readily available and can handle high throughputs (e.g., on the order of fifty pounds per hour). Furthermore, it is desirable to have a process that yields homogenous pellets which can be re-processed or injected molded to a desired shape (unlike traditional compression molding processes that are subject to variability in homogeneity, variability in bioactive glass distribution, higher likelihood of structural imperfections, have low yields, and are limited to small net shapes). Accordingly, there is a need in the art for a method of preparing a bioactive composite in which a bioactive glass, such as 45S5 or Combeite, is mixed with a polymer to produce a homogenous bioactive composite. There is also a need in the art for a method of preparing a homogeneous bioactive composite which facilitates use of various PEEK particle sizes in combination with various bioactive glass particle sizes (in which the respective particle sizes may be mis-matched). Further, there is also a need in the art for a method for preparing a bioactive composite in large batches that can be further processed to produce shaped implants that have the appropriate mechanical properties to withstand the forces required of spinal, orthopaedic and dental implants. The present invention fulfills these needs.